1. Field of the Invention
The present invention relates to a charging control method and charging control device and is suitably applied to a charging control method and charging control device for charging a secondary battery of a battery pack by means of a charging device comprising a constant-voltage constant-current circuit for example.
2. Description of the Related Art
In recent years, in the portable electronic product field such as a portable telephone and a camera built-in VTR, a secondary battery (e.g., lithium ion battery) is used as an electric source. These portable electronic products are equipped with a battery pack to store a high capacity secondary battery, and by using this battery pack as an electric source, the user can use these portable electronic products on the road.
Accordingly, in the case of using these portable electronic products, first of all it is necessary to charge the secondary battery prior to using these gears. When charging the secondary battery, if voltage and current larger than the rated value are supplied to the secondary battery, there is a fear of causing damage to said secondary battery. Thus, the charging device generally comprises a constant-voltage constant-current circuit so that the output voltage and output current larger than the rated value would not be given to the secondary battery.
Furthermore, a protection circuit is provided in the battery pack in order to prevent overcharge to the secondary battery when charging, or over-discharge from the secondary battery when discharging, and in the case where the overcharged state or the over-discharged state is detected based on the voltage value of the secondary battery, the electric path of the secondary battery is cut off and the secondary battery is protected.
At this point, a charging control method for charging the secondary battery stored in the battery pack by using said charging device is shown in FIG. 1. A charging unit 1 and a battery pack 2 are electrically connected via connecting terminals T1, T2 and start charging the secondary battery BT of the battery pack 2.
The charging unit 1 comprises a constant-voltage constant-current circuit 3 and supplies output current lower than the rated value and output voltage lower than the rated value to the secondary battery BT of the battery pack 2 by using the input voltage and the input current from the AC adapter (not shown in Figure) connected as a DC source via input terminals A and B.
In this case, the charging unit 1 supplies the output current lower than the rated value (lower than 1 coulomb) and the output voltage (reference charging voltage 4.2 volts) to the secondary battery BT and does not supply the output voltage and the output current larger than the rated value by controlling the output voltage and the output current of the constant-voltage constant-current circuit 3 by the constant-voltage constant-current control circuit 4.
More specifically, when the DC power source having the nonconforming output voltage and the output current are supplied via input terminals A and B, the input current and input voltage more than the rated value are supplied to the constant-voltage constant-current circuit 3, and as a result, heat more than the rated value is generated at the constant-voltage constant-current circuit 3 and said constant-voltage constant-current circuit 3 should be damaged. In order to prevent such damage of the constant-voltage constant-current circuit 3, the constant-voltage constant-current control circuit 4 controls the output voltage and the output current of the constant-voltage constant-current circuit 3 thus preventing the damage of said constant-voltage constant-current circuit 3.
In practice, the constant-voltage constant-current control circuit 4 is equipped with a voltage detection circuit 6 for detecting the voltage value of the output voltage to be supplied by the constant-voltage constant-current circuit 3 and outputs the voltage detection result to the control circuit 7. Moreover, in the constant-voltage constant-current control circuit 4, a current detection circuit 8 for detecting current value of the input current to be supplied by the DC power source is provided between the input terminal B and the cathode of the secondary battery BT and this circuit transmits the current detection result to the control circuit 7. The control circuit 7 stops the output of the constant-voltage constant-current circuit 3 in the case where the voltage detection result is larger than the predetermined rated voltage value, and it also stops the output of the constant-voltage constant-current circuit 3 in the case where the current detection result is larger than the predetermined rated current value.
On the other hand, a protection circuit is provided in the battery pack in order to prevent overcharge to the secondary battery BT when charging, and also a voltage detection circuit 10 for detecting the voltage value of the secondary battery BT, and a current detection circuit 11 for detecting the direction of current flowing in the electric path based on the detection of current value to be supplied to the secondary battery BT and the voltage difference between two terminals of a resistance element are provided in this battery pack 2.
In the case where the voltage detection result from the voltage detection circuit 10 shows the overcharged condition or over-discharged condition, the protection circuit 9, by turning the switching circuit 13 in OFF condition by the control circuit 12, cuts off the electric path of the battery pack 2 and stops the charging and discharging of the secondary battery BT. Thus, in the battery pack 2, the secondary battery BT is prevented from the damage by the protection circuit 9.
However, in the charging unit 1 and the battery pack 2, by setting the threshold voltage of the voltage detection circuit 6 of the charging unit 1 to become lower than the threshold value of the voltage detection circuit 10 of the battery pack 2, its output is stopped before the protection circuit 9 of the battery pack 2 when the constant-voltage constant-current circuit 3 outputs over-voltage and over-current.
In this case, as shown in FIG. 2, the reference charging voltage which the charging unit 1 supplies to the battery pack is 4.2 volts, and the detection result of the voltage detection circuit 6 at this point becomes the detection voltage range V1 (4.15-4.25 volts) including the dispersion feature (.+-.0.05 volts) of the detection circuit. Moreover, the overcharged voltage with which the protection circuit 9 of the battery pack 2 operates is 4.35 volts and the detection result of the voltage detection circuit 10 becomes the detected voltage range V2 (4.30-4.40 volts) including the dispersion feature (.+-.0.05 volts) of the detection circuit.
Accordingly, if the detection result of the voltage detection circuit 6 is within the voltage range V1 (4.15-4.25 volts) in the charging unit 1, charging should be continued as it is, but if the detection result is beyond the range of the detection voltage V1, the output of the constant-voltage constant-current circuit 3 should be stopped by the constant-voltage constant-current control circuit 4. However, in the case where the over-voltage of the detection voltage range V2 (4.30-4.40 volts) is supplied to the secondary battery BT of the battery pack due to the failure of the constant-voltage constant-current circuit 3 or the constant-voltage constant-current control circuit 4 while charging by the charging unit 1, the battery pack 2 cuts off the electric path by turning off the switching circuit 13 by the protection circuit 9 and protects the secondary battery BT.
Thus, since the voltage detection range V1 (4.15-4.25 volts) of the voltage detection circuit 6 in the charging unit 1 and the voltage detection range V2 (4.30-4.40 volts) of the voltage detection circuit 10 in the battery pack 2 are different each other, the charging unit 1 stops the output of the constant-voltage constant-current circuit 3 before the protection circuit 9 of the battery pack starts operating, smooth charging control could not be conducted before.
Accordingly, when the voltage detection circuit 6 of the charging unit 1 cannot function properly because the detection voltage range V2 (4.30-4.40 volts) with which the protection circuit of the battery pack 2 operates is higher than that of the charging unit 1, it causes a problem that the secondary battery BT should have the safety feature capable of resisting the overcharged condition (4.40 volts) corresponding to the detection voltage range V2 (4.30-4.40 volts) with which the protection circuit 9 works.
Furthermore, in this case since in the battery pack 2, the detection voltage range V2 (4.30-4.40 volts) with which the protection circuit 9 works is higher than in the charging unit 1, the sudden breakdown of the secondary battery BT could not be prevented in the case where the protection circuit 9 could not operate correctly.